A Comparative Study of Aromatization Catalysts: the Advantage of Hybrid Oxy/Carbides and Platinum-Catalysts Based on Carbon Gels

Journal of C Carbon Research

Article A Comparative Study of Aromatization Catalysts: The Advantage of Hybrid Oxy/Carbides and Platinum-Catalysts Based on Carbon Gels

Luisa M. Pastrana-Martínez * , Sergio Morales-Torres and Francisco J. Maldonado-Hódar

Department of Inorganic Chemistry, Faculty of Sciences, University of Granada, Avenida Fuente Nueva s/n, ES-18071 Granada, Spain; [email protected] (S.M.-T.); [email protected] (F.J.M.-H.) * Correspondence: [email protected]; Tel.: +34-(958)-240-443

Abstract: This manuscript is focused on the relationship between sol-gel synthesis processes and the development of new active phases with fitted morphology, porosity and surface chemistry. The influence of the above parameters on the catalytic performance of the prepared materials for the aromatization of n-hexane to benzene is also evaluated. Different series of catalysts were prepared, either using noble metals (i.e., Pt) or metal oxides (i.e., Mo, W), as active phases. In both cases, the catalytic performance and stability of classical aromatization catalysts was significantly improved. Interesting one-pot carboreduction process of the metal oxide during carbonization is suggested as a real alternative for the preparation of high-performance aromatization catalysts, leading to the formation of less acidic and non-stoichiometric oxides and carbides.

Keywords: n-hexane; benzene; carbon nanomaterials; metal active-phases; aromatization

Citation: Pastrana-Martínez, L.M.; Morales-Torres, S.; 1. Introduction Maldonado-Hódar, F.J. A Comparative Study of Aromatization Aromatic hydrocarbons are in high demand because they are the main feedstock for Catalysts: The Advantage of Hybrid the production of a wide range of products including pharmaceuticals, cosmetics, dyes Oxy/Carbides and or polymers. The aromatization of cheap and abundant light alkanes into aromatic ones Platinum-Catalysts Based on Carbon has been an important and alternative reaction to oil thermal cracking for the last 50 years, Gels. C 2021, 7, 21. https://doi.org/ allowing the preparation of high-octane gasoline blending components from naphtha, 10.3390/c7010021 among others [1–4]. The development of new catalysts for the valorization of light alkanes by transfor- Received: 22 January 2021 mation into aromatic products is an important challenge from an economic and scientific Accepted: 9 February 2021 point of view. Traditional catalysts such as Pt/Cl–Al2O3 have provided low performances Published: 12 February 2021 for light alkane (C ≤ 7) aromatization and, moreover, they can be deactivated by loss of chlorine which is formed by decomposition along a reaction of the Cl– groups present Publisher’s Note: MDPI stays neutral in the solid catalysts. In fact, chlorine is added to the feed causing corrosion and safety with regard to jurisdictional claims in hazards [5]. Zeolites have been used as alternative non-chlorinated catalysts. The best published maps and institutional affil- performance for C6 reforming to benzene was described using Pt supported on zeolite L iations. exchanged with K (Pt/KL). These catalysts showed different characteristics including weak acidity, low coke formation, shape selectivity and resistance to sintering [6,7]. In previous works, we have also identified similar factors for Pt-supported catalysts on exchanged β-zeolites [8–10]. The exchange of zeolite with alkali or alkali-earth metals, namely Cs, Copyright: © 2021 by the authors. induces textural changes, reducing the paraffin-adsorption capacity and the surface acidity, Licensee MDPI, Basel, Switzerland. minimizing the coke formation and favoring the formation of small Pt clusters with specific This article is an open access article Pt-support interactions, thus enhancing the aromatization yield. distributed under the terms and Nowadays, the optimization of aromatization catalysts continues to be of great interest conditions of the Creative Commons to the scientific community, as demonstrated by the high number of recent publications in Attribution (CC BY) license (https:// this field [5,11–14]. Reforming of light alkanes is carried out using zeolites, mainly medium creativecommons.org/licenses/by/ pore-size ZSM-5 zeolite which presents molecular shape selectivity towards benzene, 4.0/).

C 2021, 7, 21. https://doi.org/10.3390/c7010021 https://www.mdpi.com/journal/carbon C 2021, 7, 21 2 of 11

toluene and xylene (BTX). Different promoters (mainly Ga, Zn and B) are used to enhance the aromatic yield by modifying the acidity of zeolite, favoring the alkane dehydrogenation and minimizing coke deposition. In general, carbon materials present a basic character and a developed surface area and porosity, avoiding coke deposition and favoring the dispersion of supported metallic phases [15–19]. These properties could increase the performance of carbon-based catalysts in aromatization reactions. It was reported that high BTX yields can be obtained using Zn/HZSM5 catalysts for methanol aromatization, although deactivation by coking is strong. However, after regeneration, the residual of coke remaining on the zeolite surface can play an active role in the control of acidity, decreasing the acid sites’ strength and distribution, in such a way that regenerated catalysts show good BTX yields and improved stability [20]. The number of references using carbon materials to develop aromatization catalysts is quite limited. Rodríguez-Ramos [21] suggested that the aromatization of n-heptane on Pd/activated carbon catalysts is a structure-insensitive reaction. As the Pt particle size decreases, hydrogenolysis is favored, but some isomerization and aromatization remain unchanged even after doping with MgO to increase the number of basic sites. When Ru/graphite is used as catalyst, [22] the electronic density of Ru-nanoparticles is enhanced by electron transfer from graphite. This results in an increase of the activity for the n- hexane conversion, but accompanied only by a hydrogenolysis increase (aromatization levels only reached values around 10%). On the other hand, Trunschke et al. [23] pointed out that the interactions of the carbon phase with metal oxides during thermal treatments remove the acidity of the oxides, avoiding undesirable and parallel reactions (cracking). The carbon phase also favors dehydrogenation of alkanes to alkenes, which in turn leads to an increased yield of C8 aromatization into ethylbenzene (EB) and o-xylene (OX) [23]. Similar conclusions were obtained by Hoang et al. [24]. They prepared ZrO2/C composites ◦ (77.2% wt. ZrO2) pretreated at different temperatures (between 700 and 1600 C) for both C6 and C8 aromatization. In this case, the carbon phase avoids the ZrO2 sintering and induces the destruction of acid sites during thermal pretreatments. The most selective catalyst is obtained after pretreatment at 1000 ◦C, providing a benzene selectivity of 67%. Together with classical activated carbons, today there is available a large amount of advanced carbon materials (from 1D to 3D nanomaterials) with improved and fitted properties to be used in catalysis [15,17,18,25]. Among them, carbon gels (aerogels or xerogels) present additional advantages such as homogeneity and purity; moreover, its chemical and porous characteristics can be specifically designed and controlled during the sol-gel process, thus showing large possibilities in developing new catalysts [26]. Different aromatization catalysts containing several active phases such as metal oxides of Cr, Mo or W, or noble metals like Pt have been prepared by modifying their synthesis procedure [27–29]. In this manuscript, the advantage of using carbon materials in the design of new aromatization catalysts is briefly summarized. The results obtained in the n-hexane aromatization show that these easily prepared catalysts are a feasible alternative to metal- exchanged zeolitic materials, improving aspects such as stability or selectivity. Correlations between the variables of preparation (e.g., surfactants, drying, carbonization temperature), the morphology and active phases formed (non-stoichiometric oxides/carbides) and the catalytic performances are analyzed.

2. Results and Discussion Today, one of the most deeply studied systems for the aromatization of n-hexane is the metal-exchanged ZSM5 zeolite. Tshabalala et al. [30] pointed out the higher activity and stability of acid HZSM5 regarding Ga, Mo or Zn-exchanged ZSM5, where mainly Ga/ZSM5 deactivated quickly. The highest aromatization selectivity was observed for Zn/ZSM5 (around 43%) while Mo/ZSM5 exhibits the lowest selectivity values (between 20 and 25%), even worse than those observed for acid forms. More recently, Huyen [31] et al. prepared Zn/ZSM-5 and B-Zn/ZSM5 catalysts. Zn/ZSM5 is more active and selective to C 2021, 7, 21 3 of 12

C 2021, 7, 21 Ga/ZSM5 deactivated quickly. The highest aromatization selectivity was observed3 of for 11 Zn/ZSM5 (around 43%) while Mo/ZSM5 exhibits the lowest selectivity values (between 20 and 25%), even worse than those observed for acid forms. More recently, Huyen [31] et al. prepared Zn/ZSM-5 and B-Zn/ZSM5 catalysts. Zn/ZSM5 is more active and selective benzene, but deactivated faster. The selectivity reached 52% at 500 ◦C, but also decreased to benzene, but deactivated faster. The selectivity reached 52% at 500 °C, but also de- with deactivation after 3 h on stream. creased with deactivation after 3 h on stream. We have prepared different catalyst series. Figure1 summarizes the performance of We have prepared different catalyst series. Figure 1 summarizes the performance of Mo and W-carbon xerogels (prepared following the methodology b in the Section3) after Mo and W-carbon xerogels (prepared following the methodology b in the Materials and direct carbonization at 500 ◦C. The higher activity of Mo catalysts compared to the W-ones Methods section) after direct carbonization at 500 °C. The higher activity of Mo catalysts is noteworthy, and it is also slightly more selective to benzene. Conversion increased compared to the W-ones is noteworthy, and it is also slightly more selective to benzene. more or less linearly in both cases with increasing temperature, but in this sense undesired Conversion increased more or less linearly in both cases with increasing temperature, but cracking increased at the expense of aromatization. in this sense undesired cracking increased at the expense of aromatization.

FigureFigure 1.1.Conversion Conversion andand productsproductsdistribution distribution obtained obtained with with W W (opened (opened symbols) symbols) and and Mo Mo (closed symbols)(closed symbols) prepared prepared as doped as carbon doped xerogels carbon xerogels at 500 ◦C at (adapted 500 °C (adapted from ref. from [27]). ref. [27]).

TheThe physicochemicalphysicochemical propertiesproperties ofof thesethese samplessamples areare summarizedsummarized inin TableTable1 .1. Both Both carboncarbon aerogels aerogels showed showed a similar a similar pore size pore distribution, size distribution, being samples being with samples high macroporewith high 2 −1 2 −1 volumesmacropor ande volume SBET arounds and SBET 500 around m g 500, but m without g , but anywithout appreciable any appreciable mesoporosity. mesoporos- The differenceity. The difference between thebetween samples the is samples related tois therelated chemical to the composition chemical composition of their surfaces. of their In spitesurfaces. of the In fact spite that of both the fact samples that both were samples prepared were with prepared a similar metalwith a content similar in metal the starting content solution,in the starti theng total solution, metal the content total detectedmetal content by TG detected is higher by inTG the is higher case of in the the Mo case sample of the (i.e.,Mo 1.9%sample and (i.e. 1.4%, 1.9 wt.% and for Mo5001.4% wt. and for W500, Mo500 respectively). and W500, Thisrespectively) implies a. differentThis implies M–C a interaction,different M influencing–C interaction, processes influencing such asprocess lixiviationes such during as lixiviation the solvent during exchange the solvent process. ex- Itchange is also process. noteworthy It is thatalso thenoteworthy reduction that of the the Mo reduction species duringof the Mo the s carbonizationpecies during eventhe car- at ◦ 500bonizationC is favored, even at regarding 500 °C is the favored W-ones., regarding Carbon evolvedthe W-ones. as CO Cxarbonand Mo evolved (VI) reduced as COx inand a significantMo (VI) reduced proportion in a significant to Mo (V). Thisproportion deeper to gasification Mo (V). This during deeper the gasification carbonization during process the alsocarbonization increased theprocess total also metal increased content inthe this total sample metal regardingcontent in the this W500, sample where regarding tungsten the remainsW500, where as W(VI). tungsten Moreover, remains the as surface W(VI). metal Moreover content, the detected surface metal by XPS content of Mo-500 detected is also by higherXPS of thanMo-500 for is W500, also higher indicating than a for best W500, dispersion indicating (smaller a best sintering) dispersion of (small the Moerphase. sintering) of the Mo phase. Table 1. Physicochemical properties of metal-doped carbon catalysts.

Metal XPS Active S V V Sample BET macro meso (%wt.) M/C Species (m2 g−1) (cm3 g−1) (cm3 g−1) Mo (V) 82% Mo500 1.9 7.6 481 1.40 0.00 Mo (VI) 18% W500 1.4 1.4 W (VI) 100% 528 1.32 0.05

SBET = apparent surface area; Vmacro = macropore volume; Vmeso = mesopore volume. C 2021, 7, 21 4 of 11

Different nature and distributions of active sites are obtained depending on the experimental conditions used for the preparation of metal–carbon doped gels. By increasing the carbonization temperature up to 1000 ◦C, it forced the progressive reduction of the metal oxides. In this sense, a mixture of Mo (III) and Mo (V) in a ratio of 45/55 was observed for the sample Mo-1000 (results not shown) [27], however, although W remains mainly (93%) as W (VI), there is observed a certain carbidization (7% of WC). Comparing the results obtained with W-doped carbon aerogels carbonized at 500 ◦C and 1000 ◦C[27], it was pointed out that the total acidity determined by isopropanol decomposition is maintained, but strongly decreased the proportion of the propene/acetone formed in this reaction, which denotes a strong increase of basic centers regarding those acid ones with increasing carbonization temperature. This has led to a strong increase in the selectivity to benzene using W1000 regarding W500, which is enhanced from 17 to 38% at 550 ◦C. These results are in agreement with those previously published [23,24], in which the carbon phase determines the dispersion and acid character of metal oxides. In spite of the large reduction of Mo phases, the carbidization of these non-stoichiometric oxides during carbonization is not observed. It was previously suggested that Mo- ◦ carbidization requires the presence of H2 and temperatures ranging 700 C[32]. Nev- ertheless, because aromatization is carried out in the presence of H2, the formation of carbides along the reaction cannot be ruled out. In fact, Delporte et al. [33] observed the formation of carbide and oxycarbide species (MoCxOy) using MoO3 catalyst for the isomerization of n-hexane at 350 ◦C. They reported that the Mo phases formed are highly dependent of the H2/hexane ratio, determining the catalyst’s performance. In our case, when the evolution of conversion and selectivity with the time on stream were analyzed, a very stable performance (Figure2) of Mo-doped samples was observed, probably also indicating a negligible transformation of the catalyst’s surface. It is noteworthy that this stability is observed in spite of the same sample being used at different temperatures (Figure1) for around 3h. MoO x are normally used as isomerization catalysts to obtain branched alkanes, however, it is observed that in our case the isomerization is C 2021, 7, 21 negligible independent of the reaction temperature or time on stream. Cracking is the5 main of 12

process, favored by the acidity of the catalysts and the increasing reaction temperature.

FigureFigure 2.2. ConversionConversion andand productsproducts distributiondistribution obtainedobtained with with Mo500 Mo500 carbon carbon aerogel aerogel at at 550 550◦ C,°C, as as a a function of time on stream. function of time on stream.

MolybdenumMolybdenum carbide carbide catalystscatalysts have have attractedattracted thethe attention attention of of different different research research groupsgroups [[34,35]34,35] asas aromatizationaromatization catalystscatalysts ofof lightlight alkanes,alkanes, beingbeing proposedproposed asas alternativesalternatives to noble metal (Pt) catalysts. The sol-gel procedure to obtain Mo-doped carbon gels was modified and samples were prepared following the methodology (c), in the Materials and Methods section. This implies that the first RF polymerization step is carried out in agitated batch reactors with the presence/absence of surfactants, thus defining the morphology of materials (fibers, spheres, etc. can be formed). Gels are partially cured in suspension before doping with the corresponding active phase. This procedure is faster than procedure (b), avoiding large cure periods inside the glass molds, but also avoids the formation of metal nanoparticles trapped inside the organic structure of the RF organic gels. After pre-gelation, nanoparticles are forced to be formed on the surface of previously gelled RF-structures. Additionally, surfactant molecules are incorporated into the RF chemical structure forming RFS composites that offer improved anchoring sites for metal adsorption during the impregnation process [29], allowing the obtaining of a high coverage on the carbon surface of metal nanoparticles (Figure 3) even after thermal carbonization at high temperature (900 °C). C 2021, 7, 21 5 of 11

to noble metal (Pt) catalysts. The sol-gel procedure to obtain Mo-doped carbon gels was modified and samples were prepared following the methodology (c), in the Section3. This implies that the first RF polymerization step is carried out in agitated batch reactors with the presence/absence of surfactants, thus defining the morphology of materials (fibers, spheres, etc. can be formed). Gels are partially cured in suspension before doping with the corresponding active phase. This procedure is faster than procedure (b), avoiding large cure periods inside the glass molds, but also avoids the formation of metal nanoparticles trapped inside the organic structure of the RF organic gels. After pre-gelation, nanoparticles are forced to be formed on the surface of previously gelled RF-structures. Additionally, surfactant molecules are incorporated into the RF chemical structure forming RFS composites that offer improved anchoring sites for metal adsorption during the impregnation C 2021, 7, 21 process [29], allowing the obtaining of a high coverage on the carbon surface of metal6 of 12

nanoparticles (Figure3) even after thermal carbonization at high temperature (900 ◦C).

Figure 3. Highly dispersed Mo nanoparticles on carbon xerogel S10 (RF + CTAB cationic surfactant Figure 3. Highly dispersed Mo nanoparticles on carbon xerogel S10 (RF + CTAB cationic surfactant + + t-butanol) after carbonization at 900 °C. t-butanol) after carbonization at 900 ◦C.

TheThe interaction interaction of of the the RFS RFS composites composites with with the the metal metal precursor precursor is is mainly mainly determined determined byby the the nature nature of of the the surfactant. surfactant. The The stronger stronger the the RFS–Mo RFS–Mo links, links, the the deeper deeper the the reduction reduction ofof the the Mo Mo phase phase will will be be observed observed during during carbonization carbonization [ 28[28]].. The The XPS XPS analysis analysis of of samples samples preparedprepared by by this this procedure procedure indicates indicates that that up up to to 60% 60% of theof the Mo Mo detected detected in surfacein surface is forming is form- 2 likeingβ like-Mo β2-C;Mo thisC; phasethis phase is also is detectedalso detected by XRD by inXRD some in casessome [cases36]. Thus, [36]. highlyThus, highly dispersed dis- Mopersed nanoparticles Mo nanoparticles can be easily can formed be easily and carburizedformed and when carburized supported when on nanostructured supported on RFSnanostructured gels following RF anS gels easy following “one pot” an procedure, easy “one mainly pot” procedure when using, mainly a cationic when surfactant using a thatcationic interacts surfactant attractively that interacts with the attractively anionic RF with macromolecule. the anionic RF macromolecule. FigureFigure4 4shows shows the the catalyticcatalytic performanceperformance of S7 S7 catalysts, catalysts, as as an an example, example, showing showing the thetypical typical evolution evolution of this of this catalytic catalytic series series.. This Thissample sample is synthesized is synthesized in the in presence the presence of cat- ofionic cationic CTAB, CTAB, TMB TMB and and t-butanol t-butanol as as co co-surfactants,-surfactants, and and after after carbonization the the ratio ratio MoOMoOxx/MoC/MoCxx == 42/58. 42/58. As As in inthe the previous previous Mo Mo series, series, convers conversionion increased increased more more or less or less lin- linearlyearly with with temperature, temperature, isomerization isomerization is is negligible negligible and and cracking is favored withwith increas-increas- inging temperature. temperature. Due Due to to the the strong strong degree degree of of reduction reduction of of the the Mo-oxide Mo-oxide phase, phase, the the smaller smaller activityactivity of of S7 S7 regarding regarding Mo-500 Mo-500 is is noteworthy, noteworthy which, which showed showed a total a total n-hexane n-hexane conversion conversion at 580at 580◦C. °C. However, However, selectivity selectivity to benzene to benzene reached reached higher higher values values (over 60%(over at 60% 500 ◦atC). 500 In °C). these In experimentalthese experimental conditions conditions (500 ◦ C;(500 Figure °C; Figure4b), both 4b), conversion both conversion and selectivity and selectivity slowly slowly de- creaseddecreased with with time time on stream. on stream. Thus, Thus, less active less active but more but selectivemore selective phases phases are obtained are obtained by the progressiveby the progressive reduction reduction of the Mo of phasethe Mo and phase the and formation the formation of Mo oxycarbides. of Mo oxycarbides. Even when Even when increasing temperature, the selectivity at 550 °C shows values at around 50%, increasing significantly from values ranging 25% for Mo-500 catalyst (Figure 1) at this temperature. C 2021, 7, 21 6 of 11

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increasing temperature, the selectivity at 550 ◦C shows values at around 50%, increasing signiﬁcantly from values ranging 25% for Mo-500 catalyst (Figure1) at this temperature.

((aa)) ((bb))

FigureFigure 4. 4. EvolutionEvolution of of conversion conversion and and products products distribution distribution for for sample sample S7 S7:S7:: ( (aa)) as as a a function functionfunction of of temperature temperaturetemperature and and ( (bb))) as asas a aa ◦ functionfunctionfunction of ofof time timetime on onon stream streamstream at at 500 500 °°C.C.

PtPt catalysts catalysts [28] [[28]28] were were prepared prepared byby classicalclassical incipientincipient wet wetnesswetnessness impregnation impregnationimpregnation using usingusing asas support aa mesoporousmesoporous carbon carbon aerogel aerogel previously previously synthesized synthesized (procedure (procedure a). Aftera). After pretreat- pre- support a mesoporous carbon aerogel previously synthesized0 (procedure a). After pre- ments in He flow at different temperatures, only metallic Pt species0 were detected. The treatmentstreatments inin HeHe flowflow atat differentdifferent temperaturestemperatures,, onlyonly metallicmetallic PtPt0 speciesspecies werewere detected.detected. catalytic performance is related therefore only with the dispersion of metallic nanoparticles. TheThe catalyticcatalytic performanceperformance isis relatedrelated thereforetherefore onlyonly withwith thethe dispersiondispersion ofof metallicmetallic nano-nano- Figure5 summarizes the performance of C900 2Pt catalyst, which contains a 2% of Pt particles.particles. FigureFigure 55 summarizesummarizess thethe pperformanceerformance ofof C900C900 2Pt2Pt catalyst,catalyst, whichwhich concontainstains aa 2%2% (%wt.). The mean Pt particle size increased from 2.8 to 7.3 nm after pretreatment at 600 or ofof PtPt (%wt(%wt..)).. TheThe meanmean PtPt particleparticle sizesize increasedincreased fromfrom 2.82.8 toto 7.37.3 nmnm afterafter pretreatmentpretreatment atat 900 ◦C in He flow, respectively. At a glance, significant differences are observed regarding 600600 oror 900900 °°CC inin HeHe flow,flow, respectively.respectively. AtAt aa glanceglance,, significantsignificant differencesdifferences areare observedobserved the previous series using oxide/oxycarbide catalysts such as: (i) Pt catalysts are active regardingregarding thethe previousprevious seriesseries usingusing oxide/oxycarbideoxide/oxycarbide catalystscatalysts suchsuch asas:: (i)(i) PtPt catalystscatalysts areare at significantly lower temperatures; (ii) in general, benzene is the main reaction product activeactive a att significantlysignificantly lowerlower temperatemperaturestures;; ((ii)ii) inin general,general, benzenebenzene isis thethe mainmain reactionreaction obtained; (iii) significant isomerization process takes place at low temperature; and (iv) at productproduct obtainedobtained;; ((iiiiii)) significantsignificant isomerizationisomerization processprocess takestakes placeplace atat lowlow temperaturetemperature;; higher temperatures, conversion increases favoring aromatization regarding isomerization andand ((iviv)) atat higherhigher temperaturetemperaturess,, conversionconversion increasesincreases favoringfavoring aromatizationaromatization regardingregarding and cracking. isomerizationisomerization andand crackingcracking..

((aa)) ((bb))

FigureFigure 5. 5. EvolutionEvolutionEvolution ofof of conversionconversion conversion andand and productsproducts products distributiondistribution distribution forfor for samplesample sample C900C900 C900 2Pt2Pt 2Ptasas aa as functionfunction a function ofof temperaturetemperature of temperature aafterfter after pre-pretreatment in He at (a) 600 °C and (b) 900 °C. treatmentpretreatment in He in Heat (a at) 600 (a) 600°C and◦C and (b) 900 (b) 900°C. ◦C.

SmallSmall PtPt nanoparticlesnanoparticles obtainedobtained atat lowlow pretreatmentpretreatment temperaturetemperature areare highlyhighly activeactive andand nn--hexanehexane conversionconversion increasedincreased fromfrom 3535 toto 90%90% inin aa shortshort temperaturetemperature rangerange (between(between 400400 andand 465465 °C)°C);; inin thisthis case,case, aromatizationaromatization remainsremains moremore oror lessless constantconstant andand crackingcracking C 2021, 7, 21 7 of 11

C 2021, 7, 21 8 of 12 Small Pt nanoparticles obtained at low pretreatment temperature are highly active and n-hexane conversion increased from 35 to 90% in a short temperature range (between 400 and 465 ◦C); in this case, aromatization remains more or less constant and cracking increased at the expense of isomerization processes. With increases in the Pt particle size increased at the expense of isomerization processes. With increases in the Pt particle size there is a strong reduction of the catalyst activity, but aromatization is strongly favored there is a strong reduction of the catalyst activity, but aromatization is strongly favored with with increasing reaction temperature (reaching values of 78% at 500 °C), therefore de- increasing reaction temperature (reaching values of 78% at 500 ◦C), therefore decreasing in creasing in this sense both isomerization and cracking. These changes clearly point out this sense both isomerization and cracking. These changes clearly point out the structure- the structure-sensitive character of aromatization on Pt-supported catalysts. sensitive character of aromatization on Pt-supported catalysts. A similar effect was observed when increasing the Pt content from 2 to 5% (Table 2). A similar effect was observed when increasing the Pt content from 2 to 5% (Table2). Conversion decreased in spite of the higher Pt content, but benzene becomes the main Conversion decreased in spite of the higher Pt content, but benzene becomes the main product obtained, with a selectivity value of around 65%. Moreover, it is also noteworthy product obtained, with a selectivity value of around 65%. Moreover, it is also noteworthy the high stability of Pt catalysts when supported on carbon aerogels (Figure 6). These re- the high stability of Pt catalysts when supported on carbon aerogels (Figure6). These resultssults point point out out the the importance importance of of the the carbon carbon gel gel mat materialserials used to develop highlyhighly selectiveselective andand stablestable catalystscatalysts forfor thethe aromatizationaromatization ofof lightlight alkanesalkanes (n-hexane). (n-hexane).In In general,general,the themain main aromatizationaromatization productproduct detecteddetected waswas benzene,benzene, whilewhile toluenetoluene waswas notnot detected. detected. CrackingCracking leadsleads to to the the formation formation of the of the C1–C5 C1– compoundsC5 compounds and methylcyclopentane and methylcyclopentane (MCP) (MCP) was found was asfound the mainas the isomerization main isomerization product. product. Conversion Conversion wascalculated was calculated taking taking into accountinto account the amountthe amount of hexane of hexane transformed transformed and selectivityand selectivity as the as amount the amount of each of detectedeach detected product product over theover total the hexanetotal hexane consumed. consumed. Thus, Thus, the values the values of cracking of cracking selectivity selectivity presented presented include include the amountsthe amounts of the of C1–C5the C1– compoundsC5 compounds [27,28 [27,28]]. .

Table 2. Influence of the PtTable content 2. Inﬂuenceon the catalytic of the Ptperformance content on theof supported catalytic performance catalysts pretreated of supported at 600 catalysts °C. Conversion pretreated at and products distribution obtained600 ◦C. Conversion at 475 °C. and products distribution obtained at 475 ◦C. Sample d Pt C S Crack S Iso S Arom Yield Arom Sample d Pt C S Crack S Iso S Arom Yield Arom (nm) (nm)(%) (%) (%) (%) (%) (%) (%) (%) C900 2Pt C9002.8 2Pt86.8 2.8 50.7 86.8 50.74.7 44.7 4.7 38.8 44.7 38.8 C900 5Pt C9009.9 5Pt60.5 9.9 32.7 60.5 32.72.4 64.9 2.4 39.3 64.9 39.3 d Pt = Pt particle size; C = dConversion; Pt = Pt particle S Crack size; C = = Selectivity Conversion; to S Crackcracking; = Selectivity S Iso = Selectivity to cracking; to S Isoisomerization; = Selectivity toS isomerization;Arom = Selectivity to aromatization.S Arom = Selectivity to aromatization.

100 80 Conversion S-Aromatization

75 60 ) ) % ( %

( n e o n i e s c

r 50 40 n e e b v S n o C

25 20

0 0 0 50 100 150 200 250 300 Time on stream (min)

FigureFigure 6.6. EvolutionEvolution ofof conversionconversion andand benzenebenzene selectivityselectivity asas aa functionfunction ofof timetime onon streamstream atat 475475◦ C, °C, using C900 5Pt catalysts pretreated at 600 °C. using C900 5Pt catalysts pretreated at 600 ◦C. 3.3. MaterialsMaterials andand MethodsMethods CarbonCarbon gelgel supportssupports andand thethe posteriorposterior functionalizationfunctionalization withwith differentdifferent activeactive phasesphases werewere synthesized synthesized following following the the method method described described by Pekalaby Pekala [37] [37] with with different different modifications: modifications: CC2021 2021, 7, ,7 21, 21 89 ofof 11 12

a) Preparation of carbon-gel supports (Figure 7): Resorcinol (R) and formaldehyde (a) Preparation of carbon-gel supports (Figure7): Resorcinol (R) and formaldehyde (F) in (F) in a molar ratio of R/F= ½ and Na2CO3, as polymerization catalyst (C) are dis- a molar ratio of R/F= 1/2 and Na CO , as polymerization catalyst (C) are dissolved in solved in the appropriate amounts2 3 of distilled water (W). Solutions are cast into the appropriate amounts of distilled water (W). Solutions are cast into glass molds, glass molds, sealed and cured for 1 day at room temperature, 2 days at 50 °C and sealed and cured for 1 day at room temperature, 2 days at 50 ◦C and 5 days at 80 ◦C. After5 days that, gelat 80 rods °C. areAfter cut that, in pellets gel rods and are introduced cut in pellets in acetone and introduced to remove in theacetone water to insideremove the pores. the water Finally, inside gels werethe pores. supercritically Finally, g driedels were with supercritically carbon dioxide dried to obtain with the correspondingcarbon dioxide organicto obtain RF the aerogels corresponding or dried inorganic the oven RF ataerogels 120 ◦C or obtaining dried in the the correspondingoven at 120 xerogels.°C obtaining Pyrolysis the corresponding of organic gels xerogels. to obtain Pyrolysis the derivative of organic carbon gels to obtain the derivative carbon gels is carried out in a tubular furnace under N2 flow gels is carried out in a tubular furnace under N2 ﬂow by heating up to different temperaturesby heating (500 up orto 1000different◦C) withtemperatures a heating (500 rate betweenor 1000 ° 0.5C) with and 1.5a heating◦C min −rate1. between 0.5 and 1.5 °C min−1.

FigureFigure 7. 7.Sequence Sequence followed followed for for the the preparation preparation of of carbon-gel carbon-gel supports. supports.

AfterAfter obtaining obtaining thethe corresponding carbon gels, gels, supported supported catalysts catalysts were were prepared prepared by byclassical classical incipient incipient wet wet impregnation impregnation with with the the corresponding metal metal salt salt precursors precursors ([Pt ([Pt ◦ (NH(NH3)34)4]Cl]Cl22)) andand thermallythermally pretreatedpretreated inin situsitu inin inertinert (He)(He) atat temperaturestemperatures 600–900600–900 °C.C.

(a)a) MetalMetal-doped-doped carbon gels were obtainedobtained directlydirectly byby replacingreplacing NaNa22COCO33 byby thethe corre-corresponding metal metal precursor precursor in in the the starting starting solution solution [30,38] [30,38. In]. Inthis this case case,, ammonium ammonium mo- lybdatemolybdate or tungstate or tungstate was was used. used. The The res restt of ofthe the synthesis synthesis procedure procedure was was maintained maintained as inas procedure in procedure (a). (a).In such In sucha way, a way,simultaneous simultaneous carbonization carbonization of the RF of theorganic RF organic fraction andfraction decomposition and decomposition of the precursor of the precursor salts occurs salts during occurs the during same the thermal same thermal process, leadingprocess, directly leading to directly materials to materials suitable to suitable be used to in be catalysis. used in catalysis. (b)b) MetalMetal-supported-supported carbon xerogels werewere obtainedobtained inin powderpowder followingfollowing a a procedure procedure in in twotwo steps [29] [29];; firstly,firstly, thethe polymerizationpolymerization of of RF RF in in the the presence/absence presence/absence of of surfactants surfactants ◦ isis carried out in aa batchbatch reactorreactor atat 5050 °CC forfor 22 h;h;after after the the morphology morphology of of the the organic organic RF is is defined, defined, the the solid solid in in suspension suspension is isdoped doped with with metal metal precursors, precursors, then then the tem- the ◦ peraturetemperature of the of bath the bathis increased is increased up to up90 ° toC for 90 anC additional for an additional 24 h, allowing 24 h, allowing the com- pletethe complete polymerization polymerization of the organic of the fraction. organic fraction.Finally, solids Finally, are solids recovered are recovered by filtration, by ◦ properlyfiltration, washed, properly dried washed, in an dried oven inand an finally oven andcarbonized finally carbonizedat 900 °C. The at 900procedureC. The is summarizedprocedure is summarizedin Figure 8. in Figure8. C 2021, 7, 21 9 of 11 C 2021, 7, 21 10 of 12

Figure 8. Preparation of metal-doped carbon xerogel powder. Figure 8. Preparation of metal-doped carbon xerogel powder.

CatalystsCatalysts werewere properly properly texturally texturally and and chemically chemically characterized characterized using using complementary complemen- ◦ techniques,tary techniques, including including mercury mercury porosimetry porosimetry and gases and adsorptiongases adsorption (N2 at (N−1962 at −C196 and °C CO and2 ◦ atCO 0 2 atC), 0 XRD,°C), XRD, SEM, SEM, HRTEM, HRTEM, XPS, XPS, TG, etc. TG, etc. TheThe aromatizationaromatization ofof n-hexanen-hexane experimentsexperiments waswas carriedcarried outout atat atmosphericatmospheric pressurepressure usingusing aa quartzquartz micro-reactormicro-reactor withwith 0.150.15 gg catalystcatalyst formingforming aa fixedfixed bed.bed. TheThe reactorreactor waswas fedfed 3 3 −−11 withwith aa continuouscontinuous flow,flow, 60 60 cm cmmin min ,, ofof aa mixture mixture 50%He/50%H 50%He/50%H22 bubbled inin aa hexanehexane saturatorsaturator cooledcooled to to fix fix the the hexane hexane concentration concentration at 2.5%at 2.5%v/v .v/v Products. Products were were analyzed analyzed by gas by chromatography,gas chromatography, using using a flame a flame ionization ionization detector detector and a Chromosorband a Chromosorb column. column. Conversion Con- wasversion defined was ondefined the basis on the of basis the n-hexane of the n-hexane disappearance disappearance and selectivity and selectivity as the fractionas the frac- of hexanetion of hexane molecules molecules transformed transformed into each into detected each detected product. product.

4.4. ConclusionsConclusions The main drawback of aromatization catalysts is related to the loss of activity, selec- The main drawback of aromatization catalysts is related to the loss of activity, selectivity and stability, mainly due to a fast deactivation by coke. Because these reactions are tivity and stability, mainly due to a fast deactivation by coke. Because these reactions are carried out in the absence of oxygen, guaranteeing the carbon stability, the use of carbon carried out in the absence of oxygen, guaranteeing the carbon stability, the use of carbon gel-based catalysts results in evident advantages based on a developed porous texture and basicgel-based character. catalysts results in evident advantages based on a developed porous texture and Thebasic sol-gel character technology. applied to the development of an aromatization catalyst presentsThe asol different-gel technology alternative applied to optimize to the development the performance of an ofaromatization active phases, catalyst deﬁning pre- thesent nanostructure,s a different alternativ nature ande to distributionoptimize the of performance the corresponding of active active phases, phases. defining Using the non-noblenanostructure, active nature phases and (Mo, distribution W), the progressive of the corresponding reduction of active the metal phases. oxide Using phases non takes-no- placeble active simultaneously phases (Mo, to W), the the carbonization progressive of reduction the organic of the phase. metal These oxide processes phases takes minimize place thesimultaneously acidity of the to catalysts, the carbonization increasing theof the aromatization organic phase. selectivity. These Whenprocesses these minimize samples arethe synthesizedacidity of the in catalysts the presence, increasing of cationic the aromatization surfactant (CTAB), selectivity. new anchoring When these sites samples for metals are aresynthesized produced in favoring the presence a strong of dispersioncationic surfactant of the active (CTAB), phase new and anchoring a deeper degree sites for of metals reduction/carbidization.are produced favoring The a formation strong dispersion of Mo2C of again the active increases phase the and aromatization a deeper degree selectivity of re- (upduction to 60%/carbidization. in some cases) The regarding formation samples of Mo where2C again the increases active phase the remainsaromatization as metal selectiv- oxide. Inity this (up catalystto 60% in series, some the cases) strength regarding and distributionsamples where of acid the active sites determines phase remains the productas metal distribution,oxide. In this increasing catalyst series acidity, the and strength reaction and temperature, distribution favoring of acid cracking sites determines processes. the productUsing distribution, noble metals increasing (Pt) as active acidity phase, and reaction the aromatization temperature process, favor takesing cracking place on pro- the 0 metalliccesses. surface (Pt ) and the reaction strongly depends on the Pt particle size. The C6 aromatizationUsing noble is ametals structure-sensitive (Pt) as active reaction.phase, the Small aromatization nanoparticles process are take mores place active, on but the conversionmetallic surface increases (Pt0) with andtemperature, the reaction favoringstrongly cracking.depends Withon the increasing Pt particle Pt particlesize. The size, C6 conversionaromatization decreased is a structure but the-sensitive selectivity reaction. to benzene Small strongly nanoparticles increased, are reachingmore active, values but C 2021, 7, 21 10 of 11

around 70%. In this case, conversion increased favoring aromatization processes, thus obtaining high conversion and aromatization selectivity values, both aspects being stable for a long time on streams.

Author Contributions: L.M.P.-M. and S.M.-T.; investigation, L.M.P.-M. and S.M.-T.; writing—original draft preparation, F.J.M.-H.; writing—review and editing, L.M.P.-M. and S.M.-T.; supervision, F.J.M.- H.; funding acquisition, L.M.P.-M., S.M.-T. and F.J.M.-H. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Spanish Project ref. RTI 2018-099224-B100 funded by ERDF/Ministry of Science, Innovation and Universities—State Research Agency and the Nano4Fresh project (ref. PCI2020-112045), as part of the PRIMA Programme supported by the European Union. Acknowledgments: L.M.P.-M. (RYC-2016-19347) and S.M.-T. (RYC-2019-026634-I/AEI/10.13039/ 501100011033) acknowledge the Spanish Ministry of Economy and Competitiveness (MINECO), the State Research Agency and the European Social Found for their Ramón y Cajal research contracts. “Unidad de Excelencia Química Aplicada a Biomedicina y Medioambiente” of the University of Granada (UEQ-UGR) is gratefully acknowledged for the technical assistance. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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